Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining" volume 4 page 433 (Interscience Publishers, New York 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling lbs./U.S. Point .degree. F. Point .degree. F. Gal. ______________________________________ Ethyl benzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________ W-M 23.92 3.72 .+-. 0.06
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range of 10 to 32 wt. % ethyl benzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes. At present, several xylene isomerization processes are available and in commercial use.
The isomerization process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
It will be apparent that separation techniques for recovery of one or more xylene isomers will not have material effect on the ethyl benzene introduced with charge to the recovery isomerization "loop". That compound, normally present in eight carbon atom aromatic fractions, will accumulate in the loop unless excluded from the charge or converted by some reaction in the loop to products which are separable from xylenes by means tolerable in the loop. Ethyl benzene can be separated from the xylenes of boiling point near that of ethyl benzene by extremely expensive "superfractionation". This capital and operating expense cannot be tolerated in the loop where the high recycle rate would require an extremely large distillation unit for the purpose. It is a usual adjunct of low pressure, low temperature isomerization as a charge preparation facility in which ethyl benzene is separated from the virgin C.sub.8 aromatic fraction before introduction to the loop.
Other isomerization processes operate at higher pressure and temperature, usually under hydrogen pressure in the presence of catalysts which convert ethyl benzene to products readily separated by relatively simple distillation in the loop, which distillation is needed in any event to separate by-products of xylene isomerization from the recycle stream. For example, the Octafining catalyst of platinum on a silicaalumina composite exhibits the dual functions of hydrogenation/dehydrogenation and isomerization.
Ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethylene and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has but a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relates to % approach to equilibrium. Products formed from ethyl benzene include C.sub.6.sup.+ naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst avtivity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
A different approach to conversion of ethyl benzene is described in Morrison U.S. Pat. No. 3,856,872, dated Dec. 24, 1974. Over an active acid catalyst typified by zeolite ZSM-5, ethyl benzene disproportionates to benzene and diethyl benzene which are readily separated from xylenes by the distillation equipment needed in the loop to remove by-products. It is recognized that rate of disproportionation of ethyl benzene is related to the rate of conversion of xylenes to other compounds, e.g. by disproportionation.
In the known processes for accepting ethyl benzene to the loop, conversion of that compound is constrained by the need to hold conversion of xylenes to an acceptable level. Thus, although the Morrison technique provides significant advantages over Octafining in this respect, operating conditions are still selected to balance the advantages of ethyl benzene conversion against the disadvantages of xylene loss by disproportionation and the like.
The present invention is predicated on discovery of a combination of catalyst and operating conditions which decouples ethyl benzene conversion from xylene loss in a xylene isomerization reaction, thus permitting feed of C.sub.8 fractions which contain ethyl benzene without sacrifice of xylenes to conditions which will promote adequate conversion of ethyl benzene.